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# OCR Chemistry A Exam Thread (Breadth - May 27 2016 and Depth - June 10 2016) watch

1. URGENT QUESTION!!!
What are the SI units to use in the Pv=nRT equation?? i have seen so many different websites and textbooks say different things. aaaaargh. I thought it was
P= Pa (not KPa)
v= m3 (not dm3)
n is obviously just n, same as R and T is definitely in Kelvin.
Please someone help. Its confusing me and annoying me because the equation isn't hard.
2. (Original post by asinghj)
Electronegativity is the attraction of a atom to get the shared pair of electrons, for which you need to know how it changes (increases going up and increases going from left to right)

Periodicity is the repeating trends across each period of the periodic table (you need to know first ionisation energy increases across a period, the trend in melting and boiling point and atomic radius)

Formation of pi bonds I have no clue so help?
Thanks.

I just had look and pi bonds are formed after a sigma bond has been formed and is formed by the p-orbitals overlapping above and below the carbon atoms
3. (Original post by maxross)
URGENT QUESTION!!!
What are the SI units to use in the Pv=nRT equation?? i have seen so many different websites and textbooks say different things. aaaaargh. I thought it was
P= Pa (not KPa)
v= m3 (not dm3)
n is obviously just n, same as R and T is definitely in Kelvin.
Please someone help. Its confusing me and annoying me because the equation isn't hard.
m3 and pa
4. (Original post by maxross)
URGENT QUESTION!!!
What are the SI units to use in the Pv=nRT equation?? i have seen so many different websites and textbooks say different things. aaaaargh. I thought it was
P= Pa (not KPa)
v= m3 (not dm3)
n is obviously just n, same as R and T is definitely in Kelvin.
Please someone help. Its confusing me and annoying me because the equation isn't hard.
yh the ones you mention are right
5. (Original post by 4nonymous)
yh the ones you mention are right
Thank you guys. I though they were but some questions i have seen use dm3 and KPa?? oh well, in the exam today ill use m3 and Pa.

Thank you so much
6. (Original post by maxross)
URGENT QUESTION!!!
What are the SI units to use in the Pv=nRT equation?? i have seen so many different websites and textbooks say different things. aaaaargh. I thought it was
P= Pa (not KPa)
v= m3 (not dm3)
n is obviously just n, same as R and T is definitely in Kelvin.
Please someone help. Its confusing me and annoying me because the equation isn't hard.
Pressure is Pa (even though Pa is technically not SI units, it's derived from Jm^-3)
Volume is m^3 (that is definitely from SI units)
7. (Original post by maxross)
Thank you guys. I though they were but some questions i have seen use dm3 and KPa?? oh well, in the exam today ill use m3 and Pa.

Thank you so much
Don't want to confuse you so you might not want to read this:
If you use kPa that is derived from Jdm^-3 so for volume you would use dm^3. it's technically not wrong.. But stick to Pa and m^3
8. (Original post by asinghj)
Pressure is Pa (even though Pa is technically not SI units, it's derived from Jm^-3)
Volume is m^3 (that is definitely from SI units)
QUESTION :

1.68g noble gas occupies volume of 500cm3 at RTP (298k and 100KPa). Identify noble gas.

9. Hey you know for the enthalpy calculations? When you're working out a question using formation values in the table is it product - reactants, and combustion values reactants - products and change of reaction/ bond enthalpy products - reactants??? I always get confused on which one is which
10. (Original post by maxross)
QUESTION :

1.68g noble gas occupies volume of 500cm3 at RTP (298k and 100KPa). Identify noble gas.

Ok that is using RTP, which is not 298K and 100kPa (that is STP)

At RTP, n = volume / molar volume
So n = 500 / 24000 = 0.0208 mol
Also, n= mass / molar mass
So molar mass = mass / n
So Mr = 1.68 / 0.0208 = 80.77

Since it's a noble gas, it must be krypton (closest molar mass)

This is how I would do it but for some reason it doesn't seem right, anyone?
11. (Original post by snickercell)
Hey you know for the enthalpy calculations? When you're working out a question using formation values in the table is it product - reactants, and combustion values reactants - products and change of reaction/ bond enthalpy products - reactants??? I always get confused on which one is which

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12. (Original post by maxross)
QUESTION :

1.68g noble gas occupies volume of 500cm3 at RTP (298k and 100KPa). Identify noble gas.

500/24000 = 0.0208 mol (3sf)

1.68 / 0.0208 = 80.8 g mol-1

Kr due to different isotopic abundances
13. Could someone explain why pent-1-ene does not show stereoisomerism please?
14. (Original post by asinghj)
Ok that is using RTP, which is not 298K and 100kPa (that is STP)

At RTP, n = volume / molar volume
So n = 500 / 24000 = 0.0208 mol
Also, n= mass / molar mass
So molar mass = mass / n
So Mr = 1.68 / 0.0208 = 80.77

Since it's a noble gas, it must be krypton (closest molar mass)

This is how I would do it but for some reason it doesn't seem right, anyone?
that is correct answer in book. but they have worked it out using Pv=nRT
15. (Original post by maxross)
that is correct answer in book. but they have worked it out using Pv=nRT
1 mol of gas occupies 24.0 dm3 at RTP (approximately)
16. (Original post by MissPR)
Could someone explain why pent-1-ene does not show stereoisomerism please?
On one of the carbons in C=C there are the same groups (2 hydrogens) and therefore, there is no priority group
17. (Original post by asinghj)
On one of the carbons in C=C there are the same groups (2 hydrogens) and therefore, there is no priority group
Thank youu
18. (Original post by maxross)
that is correct answer in book. but they have worked it out using Pv=nRT
From what I've learnt, for gases at RTP, use n= volume / molar volume
19. (Original post by asinghj)
From what I've learnt, for gases at RTP, use n= volume / molar volume
nice.

what you recon'll come up today???
20. (Original post by snickercell)
Hey you know for the enthalpy calculations? When you're working out a question using formation values in the table is it product - reactants, and combustion values reactants - products and change of reaction/ bond enthalpy products - reactants??? I always get confused on which one is which
I think its always reactants - products

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